In no particular order (top of my head and mostly considering the desktop Windows one)
- The history (scroll up) takes a lot to load (this is on 300mbit up/down and fast processor)
- html content in the chat totally hogs the processor
- Attachments takes forever to open internally. Not so large files like 4-5MB of log pretty much wreck it
- upload is hilariously slow (again 300mbit up/down)
- loading in browser requires an explicit login to microsoft account to open, instead of a generated link that actually works (to add some salt, my company requires 2FA for the MS login)
- audio quality issues, 'intelligent' (echo blocking algorithm kicks in wherever it wants), call drops. Esp. bad in group calls
- 'teams' chat gotta be a joke with each post forming its own thread, and you'd need to click to answer the proper one. Close to unusable feature that bears the name of the product
- Video sharing has a massive lag on screen sharing both on visual and startup time
- Blurred background algorithm is a massive flicker and it really resents people with green (t-)shirts
- Calling the phone, instead the desktop while working on the desktop. (Half of my team uses phone version as it's less buggy)
- One more: if you don't login the phone app, it will remind you to login every 3hours - until it gets uninstalled....
I can rant for at least 45minutes about it... and it's the 'default' chat application the company uses for years (along with Lync/Skype for business before) and I use it as my 'main' chat application.